Characterization of bile acid transport mediated by multidrug resistance associated protein 2 and bile salt export pump

Preincubation Time-Dependent, Long-Lasting Inhibition of Drug Transporters and Impact on the Prediction of Drug-Drug Interactions

Transporter-mediated drug-drug interaction (DDI) is of clinical concern, and the quantitative prediction of DDIs is an indispensable part of drug development. Cell-based inhibition assays, in which a representative probe substrate and a potential inhibitor are coincubated, are routinely performed to assess the inhibitory potential of new molecular entities on drug transporters. However, the inhibitory effect of cyclosporine A (CsA) on organic anion transporting polypeptide (OATP) 1B1 is substantially potentiated with CsA preincubation, and this effect is both long-lasting and dependent on the preincubation time. This phenomenon has also been reported with transporters other than OATP1Bs, but it is considered more prevalent among OATP1Bs and organic cation transporters. Regulatory agencies have also noted this preincubation effect and have recommended that pharmaceutical companies consider inhibitor preincubation when performing in vitro OATP1B1 and OATP1B3 inhibition studies. Although the underlying mechanisms responsible for the preincubation effect are not fully understood, a trans-inhibition mechanism was recently demonstrated for OATP1B1 inhibition by CsA, in which CsA inhibited OATP1B1 not only extracellularly (cis-inhibition) but also intracellularly (trans-inhibition). Furthermore, the trans-inhibition potency of CsA was much greater than that of cis-inhibition, suggesting that trans-inhibition might be a key driver of clinical DDIs of CsA with OATP1B substrate drugs. Although confidence in transporter-mediated DDI prediction is generally considered to be low, the predictability might be further improved by incorporating the trans-inhibition mechanism into static and dynamic models for preincubation-dependent inhibitors of OATP1Bs and perhaps other transporters. SIGNIFICANCE STATEMENT: Preincubation time-dependent, long-lasting inhibition has been observed for OATP1B1 and other solute carrier transporters in vitro. Recently, a trans-inhibition mechanism for the preincubation effect of CsA on OATP1B1 inhibition was identified, with the trans-inhibition potency being greater than that of cis-inhibition. The concept of trans-inhibition may allow us to further understand the mechanism of transporter-mediated DDIs not only for OATP1B1 but also for other transporters and to improve the accuracy and confidence of DDI predictions.

Bile acid metabolism and signaling: Emerging pharmacological targets of dietary polyphenols

Beyond their role as emulsifiers of lipophilic compounds, bile acids (BAs) are signaling endocrine molecules that show differential affinity and specificity for a variety of canonical and non-canonical BA receptors. Primary BAs (PBAs) are synthesized in the liver while secondary BAs (SBAs) are gut microbial metabolites of PBA species. PBAs and SBAs signal to BA receptors that regulate downstream pathways of inflammation and energy metabolism. Dysregulation of BA metabolism or signaling has emerged as a feature of chronic disease. Dietary polyphenols are non-nutritive plant-derived compounds associated with decreased risk of metabolic syndrome, type-2 diabetes, hepatobiliary and cardiovascular disease. Evidence suggests that the health promoting effects of dietary polyphenols are linked to their ability to alter the gut microbial community, the BA pool, and BA signaling. In this review we provide an overview of BA metabolism and summarize studies that link the cardiometabolic improvements of dietary polyphenols to their modulation of BA metabolism and signaling pathways, and the gut microbiota. Finally, we discuss approaches and challenges in deciphering cause-effect relationships between dietary polyphenols, BAs, and gut microbes.

Role of Hepatocyte Transporters in Drug-Induced Liver Injury (DILI)-In Vitro Testing

Bile acids and bile salts (BA/BS) are substrates of both influx and efflux transporters on hepatocytes. Canalicular efflux transporters, such as BSEP and MRP2, are crucial for the removal of BA/BS to the bile. Basolateral influx transporters, such as NTCP, OATP1B1/1B3, and OSTα/β, cooperate with canalicular transporters in the transcellular vectorial flux of BA/BS from the sinusoids to the bile. The blockage of canalicular transporters not only impairs the bile flow but also causes the intracellular accumulation of BA/BS in hepatocytes that contributes to, or even triggers, liver injury. In the case of BA/BS overload, the efflux of these toxic substances back to the blood via MRP3, MRP4, and OST α/β is considered a relief function. FXR, a key regulator of defense against BA/BS toxicity suppresses de novo bile acid synthesis and bile acid uptake, and promotes bile acid removal via increased efflux. In drug development, the early testing of the inhibition of these transporters, BSEP in particular, is important to flag compounds that could potentially inflict drug-induced liver injury (DILI). In vitro test systems for efflux transporters employ membrane vesicles, whereas those for influx transporters employ whole cells. Additional in vitro pharmaceutical testing panels usually include cellular toxicity tests using hepatocytes, as well as assessments of the mitochondrial toxicity and accumulation of reactive oxygen species (ROS). Primary hepatocytes are the cells of choice for toxicity testing, with HepaRG cells emerging as an alternative. Inhibition of the FXR function is also included in some testing panels. The molecular weight and hydrophobicity of the drug, as well as the steady-state total plasma levels, may positively correlate with the DILI potential. Depending on the phase of drug development, the physicochemical properties, dosing, and cut-off values of BSEP IC₅₀ ≤ 25-50 µM or total C_ss,plasma/BSEP IC₅₀ ≥ 0.1 may be an indication for further testing to minimize the risk of DILI liability.

Bile acids as inflammatory mediators and modulators of intestinal permeability

Bile acids are critical for the digestion and absorption of lipids and fat-soluble vitamins; however, evidence continues to emerge supporting additional roles for bile acids as signaling molecules. After they are synthesized from cholesterol in the liver, primary bile acids are modified into secondary bile acids by gut flora contributing to a diverse pool and making the composition of bile acids highly sensitive to alterations in gut microbiota. Disturbances in bile acid homeostasis have been observed in patients with Inflammatory Bowel Diseases (IBD). In fact, a decrease in secondary bile acids was shown to occur because of IBD-associated dysbiosis. Further, the increase in luminal bile acids due to malabsorption in Crohn's ileitis and ileal resection has been implicated in the induction of diarrhea and the exacerbation of inflammation. A causal link between bile acid signaling and intestinal inflammation has been recently suggested. With respect to potential mechanisms related to bile acids and IBD, several studies have provided strong evidence for direct effects of bile acids on intestinal permeability in porcine and rodent models as well as in humans. Interestingly, different bile acids were shown to exert distinct effects on the inflammatory response and intestinal permeability that require careful consideration. Such findings revealed a potential effect for changes in the relative abundance of different bile acids on the induction of inflammation by bile acids and the development of IBD. This review summarizes current knowledge about the roles for bile acids as inflammatory mediators and modulators of intestinal permeability mainly in the context of inflammatory bowel diseases.

Hepatotoxicity of the Major Anthraquinones Derived From Polygoni Multiflori Radix Based on Bile Acid Homeostasis

Polygoni Multiflori Radix (PMR), the dried root of Polygonum Multiflorum Thunb., has been widely used as traditional Chinese medicines in clinical practice for centuries. However, the frequently reported hepatotoxic adverse effects hindered its safe use in clinical practice. This study aims to explore the hepatotoxic effect of PMR extract and the major PMR derived anthraquinones including emodin, chrysophanol, and physcion in mice and the underlying mechanisms based on bile acid homeostasis. After consecutively treating the ICR mice with PMR extract or individual anthraquinones for 14 or 28 days, the liver function was evaluated by measuring serum enzymes levels and liver histological examination. The compositions of bile acids (BAs) in the bile, liver, and plasma were measured by LC-MS/MS, followed by Principal Component Analysis (PCA) and Partial Least Squares Discriminate Analysis (PLS-DA). Additionally, gene and protein expressions of BA efflux transporters, bile salt export pump (Bsep) and multidrug resistance-associated protein 2 (Mrp2), were examined to investigate the underlying mechanisms. After 14-day administration, mild inflammatory cell infiltration in the liver was observed in the physcion- and PMR-treated groups, while it was found in all the treated groups after 28-day treatment. Physcion and PMR extract induced hepatic BA accumulation after 14-day treatment, but such accumulation was attenuated after 28-day treatment. Based on the PLS-DA results, physcion- and PMR-treated groups were partially overlapping and both groups showed a clear separation with the control group in the mouse liver. The expression of Bsep and Mrp2 in the physcion- and PMR-treated mouse liver was decreased after 14-day treatment, while the downregulation was abrogated after 28-day treatment. Our study, for the first time, demonstrated that both PMR extract and tested anthraquinones could alter the disposition of either the total or individual BAs in the mouse bile, liver, and plasma via regulating the BA efflux transporters and induce liver injury, which provide a theoretical basis for the quality control and safe use of PMR in practice.

Multidrug Resistance-Associated Protein 2 Deficiency Aggravates Estrogen-Induced Impairment of Bile Acid Metabolomics in Rats

Multidrug resistance-associated protein 2 (Mrp2) mediates biliary secretion of anionic endobiotics and xenobiotics. Genetic alteration of Mrp2 leads to conjugated hyperbilirubinemia and predisposes to the development of intrahepatic cholestasis of pregnancy (ICP), characterized by increased plasma bile acids (BAs) due to mechanisms that are incompletely understood. Therefore, this study aimed to characterize BA metabolomics during experimental Mrp2 deficiency and ICP. ICP was modeled by ethinylestradiol (EE) administration to Mrp2-deficient (TR) rats and their wild-type (WT) controls. Spectra of BAs were analyzed in plasma, bile, and stool using an advanced liquid chromatography–mass spectrometry (LC–MS) method. Changes in BA-related genes and proteins were analyzed in the liver and intestine. Vehicle-administered TR rats demonstrated higher plasma BA concentrations consistent with reduced BA biliary secretion and increased BA efflux from hepatocytes to blood via upregulated multidrug resistance-associated protein 3 (Mrp3) and multidrug resistance-associated protein 4 (Mrp4) transporters. TR rats also showed a decrease in intestinal BA reabsorption due to reduced ileal sodium/bile acid cotransporter (Asbt) expression. Analysis of regulatory mechanisms indicated that activation of the hepatic constitutive androstane receptor (CAR)-Nuclear factor erythroid 2-related factor 2 (Nrf2) pathway by accumulating bilirubin may be responsible for changes in BA metabolomics in TR rats. Ethinylestradiol administration to TR rats further increased plasma BA concentrations as a result of reduced BA uptake and increased efflux via reduced Slco1a1 and upregulated Mrp4 transporters. These results demonstrate that Mrp2-deficient organism is more sensitive to estrogen-induced cholestasis. Inherited deficiency in Mrp2 is associated with activation of Mrp3 and Mrp4 proteins, which is further accentuated by increased estrogen. Bile acid monitoring is therefore highly desirable in pregnant women with conjugated hyperbilirubinemia for early detection of intrahepatic cholestasis.

The Role of Vitamin K in Cholestatic Liver Disease

Vitamin K (VK) is a ligand of the pregnane X receptor (PXR), which plays a critical role in the detoxification of xenobiotics and metabolism of bile acids. VK₁ may reduce the risk of death in patients with chronic liver failure. VK deficiency is associated with intrahepatic cholestasis, and is already being used as a drug for cholestasis-induced liver fibrosis in China. In Japan, to treat osteoporosis in patients with primary biliary cholangitis, VK₂ formulations are prescribed, along with vitamin D₃. Animal studies have revealed that after bile duct ligation-induced cholestasis, PXR knockout mice manifested more hepatic damage than wild-type mice. Ligand-mediated activation of PXR improves biochemical parameters. Rifampicin is a well-known human PXR ligand that has been used to treat intractable pruritus in severe cholestasis. In addition to its anti-cholestatic properties, PXR has anti-fibrotic and anti-inflammatory effects. However, because of the scarcity of animal studies, the mechanism of the effect of VK on cholestasis-related liver disease has not yet been revealed. Moreover, the application of VK in cholestasis-related diseases is controversial. Considering this background, the present review focuses on the effect of VK in cholestasis-related diseases, emphasizing its function as a modulator of PXR.

Dubin-Johnson Syndrome as Differential Diagnosis for Neonatal Cholestasis

OBJECTIVES

Dubin-Johnson syndrome (DJS) is an autosomal recessive disorder in which multidrug-resistance-associated protein 2 (MRP2) deficiency causes an excretion disorder of conjugated bilirubin from hepatocytes into bile canaliculi. Its clinical presentation as neonatal cholestasis (NC) is rare but represents an important differential diagnosis. We aimed to define DJS-specific characteristics in NC, in particular in contrast to biliary atresia (BA) patients, and to highlight diagnostic tools that can help to avoid invasive diagnostic tests.

METHODS

We performed a review of case records from 2006 to 2020 and compared 4 DJS patients to 26 patients with proven BA consecutively diagnosed from 2014 to 2017. DJS was diagnosed by urine coproporphyrin analysis (UCA) and by genetic analysis (GA) for disease-associated ABCC2 variants.

RESULTS

Four male patients with NC were diagnosed with DJS by UCA and GA. DJS patients presenting as NC showed significantly lower values for aspartate aminotransferase (AST) (P < 0.001), for alanine aminotransferase (ALT) (P = 0.002) and for gamma-glutamyl transferase (GGT) (P < 0.001) compared with BA patients. Other examinations, however, could not clearly discriminate them (e.g.: stool colour, serum bile acids, total serum bilirubin).

CONCLUSIONS

DJS is not only a rare differential diagnosis in NC with a suspicious phenotype (almost normal AST, ALT) but also shows overlapping features with BA. It should, therefore, be considered in every infant with NC and an atypical liver enzyme pattern to protect patients from unnecessary, invasive examinations. For this, UCA is a fast and reliable diagnostic tool. Confirmation based on GA is recommended. DJS patients have a good long-term prognosis.

Long-term trans-inhibition of the hepatitis B and D virus receptor NTCP by taurolithocholic acid

Human hepatic bile acid transporter Na⁺/taurocholate cotransporting polypeptide (NTCP) represents the liver-specific entry receptor for the hepatitis B and D viruses (HBV/HDV). Chronic hepatitis B and D affect several million people worldwide, but treatment options are limited. Recently, HBV/HDV entry inhibitors targeting NTCP have emerged as promising novel drug candidates. Nevertheless, the exact molecular mechanism that NTCP uses to mediate virus binding and entry into hepatocytes is still not completely understood. It is already known that human NTCP mRNA expression is downregulated under cholestasis. Furthermore, incubation of rat hepatocytes with the secondary bile acid taurolithocholic acid (TLC) triggers internalization of the rat Ntcp protein from the plasma membrane. In the present study, the long-term inhibitory effect of TLC on transport function, HBV/HDV receptor function, and membrane expression of human NTCP were analyzed in HepG2 and human embryonic kidney (HEK293) cells stably overexpressing NTCP. Even after short-pulse preincubation, TLC had a significant long-lasting inhibitory effect on the transport function of NTCP, but the NTCP protein was still present at the plasma membrane. Furthermore, binding of the HBV/HDV myr-preS1 peptide and susceptibility for in vitro HDV infection were significantly reduced by TLC preincubation. We hypothesize that TLC rapidly accumulates in hepatocytes and mediates long-lasting trans-inhibition of the transport and receptor function of NTCP via a particular TLC-binding site at an intracellularly accessible domain of NTCP. Physiologically, this trans-inhibition might protect hepatocytes from toxic overload of bile acids. Pharmacologically, it provides an interesting novel NTCP target site for potential long-acting HBV/HDV entry inhibitors.NEW & NOTEWORTHY The hepatic bile acid transporter NTCP is a high-affinity receptor for hepatitis B and D viruses. This study shows that TLC rapidly accumulates in NTCP-expressing hepatoma cells and mediates long-lasting trans-inhibition of NTCP's transporter and receptor function via an intracellularly accessible domain, without substantially affecting its membrane expression. This domain is a promising novel NTCP target site for pharmacological long-acting HBV/HDV entry inhibitors.

Effects of patent ductus venosus on bile acid homeostasis in aryl hydrocarbon receptor (AhR)-null mice

The Aryl hydrocarbon receptor (AhR) is primarily known as one of the xenosensors and regulators of drug-metabolizing genes. Bile acids (BAs) are synthesized in the liver, and undergo several enterohepatic recirculations in which the liver removes BAs from the portal blood, minimizing the BAs that spill over into the systemic circulation. Previous studies revealed a lifelong patent ductus venosus (PDV) in AhR-null mice. Increased concentration of total BAs (Σ-BAs) in AhR-null mice is known; however, the impact of PDV on BA homeostasis in liver and bile remains unclear. This work investigated the consequences of PDV on BA homeostasis by comparing AhR-null and wild-type (WT) mice of both genders. In serum, Σ-BAs were markedly higher (64-85-fold) in AhR-null mice than in WT mice, especially due to the increase of tri-OH primary BAs (86-142-fold). Despite the extremely high concentration of serum BAs, the concentration of BAs in livers of AhR-null mice remained similar to WT mice. AhR-null livers were protected against increased BA influx by downregulation of uptake transporters and BA synthetic enzymes in the alternative pathway. Although livers of AhR-null mice are 20-25% smaller than WT mice, biliary excretion of BAs was maintained in the AhR-null mice, and even tended to increase. Surprisingly, intestinal Fgf15 expression was not increased, even though there was a marked increase in serum BA concentrations. Although PDV resulted in extremely high BA concentrations in serum of AhR-null mice, they maintained a concentration of BAs in liver and biliary excretion of BAs similar to control mice.

The Systems Biology of Drug Metabolizing Enzymes and Transporters: Relevance to Quantitative Systems Pharmacology

Quantitative systems pharmacology (QSP) has emerged as a transformative science in drug discovery and development. It is now time to fully rethink the biological functions of drug metabolizing enzymes (DMEs) and transporters within the framework of QSP models. The large set of DME and transporter genes are generally considered from the perspective of the absorption, distribution, metabolism, and excretion (ADME) of drugs. However, there is a growing amount of data on the endogenous physiology of DMEs and transporters. Recent studies—including systems biology analyses of “omics” data as well as metabolomics studies—indicate that these enzymes and transporters, which are often among the most highly expressed genes in tissues like liver, kidney, and intestine, have coordinated roles in fundamental biological processes. Multispecific DMEs and transporters work together with oligospecific and monospecific ADME proteins in a large multiorgan remote sensing and signaling network. We use the Remote Sensing and Signaling Theory (RSST) to examine the roles of DMEs and transporters in intratissue, interorgan, and interorganismal communication via metabolites and signaling molecules. This RSST‐based view is applicable to bile acids, uric acid, eicosanoids, fatty acids, uremic toxins, and gut microbiome products, among other small organic molecules of physiological interest. Rooting this broader perspective of DMEs and transporters within QSP may facilitate an improved understanding of fundamental biology, physiologically based pharmacokinetics, and the prediction of drug toxicities based upon the interplay of these ADME proteins with key pathways in metabolism and signaling. The RSST‐based view should also enable more tailored pharmacotherapy in the setting of kidney disease, liver disease, metabolic syndrome, and diabetes. We further discuss the pharmaceutical and regulatory implications of this revised view through the lens of systems physiology.

Primary biliary cholangitis: pathogenesis and therapeutic opportunities

Primary biliary cholangitis is a chronic, seropositive and female-predominant inflammatory and cholestatic liver disease, which has a variable rate of progression towards biliary cirrhosis. Substantial progress has been made in patient risk stratification with the goal of personalized care, including early adoption of next-generation therapy with licensed use of obeticholic acid or off-label fibrate derivatives for those with insufficient benefit from ursodeoxycholic acid, the current first-line drug. The disease biology spans genetic risk, epigenetic changes, dysregulated mucosal immunity and altered biliary epithelial cell function, all of which interact and arise in the context of ill-defined environmental triggers. A current focus of research on nuclear receptor pathway modulation that specifically and potently improves biliary excretion, reduces inflammation and attenuates fibrosis is redefining therapy. Patients are benefiting from pharmacological agonists of farnesoid X receptor and peroxisome proliferator-activated receptors. Immunotherapy remains a challenge, with a lack of target definition, pleiotropic immune pathways and an interplay between hepatic immune responses and cholestasis, wherein bile acid-induced inflammation and fibrosis are dominant clinically. The management of patient symptoms, particularly pruritus, is a notable goal reflected in the development of rational therapy with apical sodium-dependent bile acid transporter inhibitors.

Current insights in the complexities underlying drug-induced cholestasis

Drug-induced cholestasis (DIC) poses a major challenge to the pharmaceutical industry and regulatory agencies. It causes both drug attrition and post-approval withdrawal of drugs. DIC represents itself as an impaired secretion and flow of bile, leading to the pathological hepatic and/or systemic accumulation of bile acids (BAs) and their conjugate bile salts. Due to the high number of mechanisms underlying DIC, predicting a compound's cholestatic potential during early stages of drug development remains elusive. A profound understanding of the different molecular mechanisms of DIC is, therefore, of utmost importance. Although many knowledge gaps and caveats still exist, it is generally accepted that alterations of certain hepatobiliary membrane transporters and changes in hepatocellular morphology may cause DIC. Consequently, liver models, which represent most of these mechanisms, are valuable tools to predict human DIC. Some of these models, such as membrane-based in vitro models, are exceptionally well-suited to investigate specific mechanisms (i.e. transporter inhibition) of DIC, while others, such as liver slices, encompass all relevant biological processes and, therefore, offer a better representation of the in vivo situation. In the current review, we highlight the principal molecular mechanisms associated with DIC and offer an overview and critical appraisal of the different liver models that are currently being used to predict the cholestatic potential of drugs.

Altered bile acid composition and disposition in a mouse model of non-alcoholic steatohepatitis

Non-alcoholic steatohepatitis (NASH) is a progressive inflammatory and fibrotic disease. However, the progression mechanism of NASH is not well understood. Bile acids are endogenous molecules that regulate cholesterol homeostasis, lipid solubilization in the intestinal lumen, and metabolic signaling via several receptors. In this study, we investigated the relationship between bile acid composition and NASH-associated fibrosis using a mouse model fed choline-deficient, L-amino-acid-defined, high-fat diet with 0.1% methionine (CDAHFD). C57BL/6 J mice fed CDAHFD developed NASH and fibrosis within few weeks. With the progress of NASH-associated liver fibrosis, altered bile acid composition was observed in the liver, bile, and peripheral plasma. Decreased mRNA levels of bile acid metabolizing enzymes such as Cyp7a1 and Baat were observed in contrast to increased Sult2a1 level in the liver. Increased mRNA levels of Ostβ and Abcc4 and decreased in mRNA levels of Bsep, Abcc2, Ntcp, and Oatp1b2, suggesting that bile acids efflux from hepatocytes into the peripheral plasma rather than into bile. In conclusion, the changes in bile acid metabolizing enzymes and transporters expression, resulting in increasing the total bile acid concentration in the plasma, signify a protection mechanism by the hepatocyte to reduce hepatotoxicity during disease progression to NASH but may promote liver fibrosis.

The UDP-Glycosyltransferase (UGT) Superfamily: New Members, New Functions, and Novel Paradigms

UDP-glycosyltransferases (UGTs) catalyze the covalent addition of sugars to a broad range of lipophilic molecules. This biotransformation plays a critical role in elimination of a broad range of exogenous chemicals and by-products of endogenous metabolism, and also controls the levels and distribution of many endogenous signaling molecules. In mammals, the superfamily comprises four families: UGT1, UGT2, UGT3, and UGT8. UGT1 and UGT2 enzymes have important roles in pharmacology and toxicology including contributing to interindividual differences in drug disposition as well as to cancer risk. These UGTs are highly expressed in organs of detoxification (e.g., liver, kidney, intestine) and can be induced by pathways that sense demand for detoxification and for modulation of endobiotic signaling molecules. The functions of the UGT3 and UGT8 family enzymes have only been characterized relatively recently; these enzymes show different UDP-sugar preferences to that of UGT1 and UGT2 enzymes, and to date, their contributions to drug metabolism appear to be relatively minor. This review summarizes and provides critical analysis of the current state of research into all four families of UGT enzymes. Key areas discussed include the roles of UGTs in drug metabolism, cancer risk, and regulation of signaling, as well as the transcriptional and posttranscriptional control of UGT expression and function. The latter part of this review provides an in-depth analysis of the known and predicted functions of UGT3 and UGT8 enzymes, focused on their likely roles in modulation of levels of endogenous signaling pathways.

Pharmacological Activation of PXR and CAR Downregulates Distinct Bile Acid-Metabolizing Intestinal Bacteria and Alters Bile Acid Homeostasis

The gut microbiome regulates important host metabolic pathways including xenobiotic metabolism and intermediary metabolism, such as the conversion of primary bile acids (BAs) into secondary BAs. The nuclear receptors pregnane X receptor (PXR) and constitutive androstane receptor (CAR) are well-known regulators for xenobiotic biotransformation in liver. However, little is known regarding the potential effects of PXR and CAR on the composition and function of the gut microbiome. To test our hypothesis that activation of PXR and CAR regulates gut microbiota and secondary BA synthesis, 9-week-old male conventional and germ-free mice were orally gavaged with corn oil, PXR agonist PCN (75 mg/kg), or CAR agonist TCPOBOP (3 mg/kg) once daily for 4 days. PCN and TCPOBOP decreased two taxa in the Bifidobacterium genus, which corresponded with decreased gene abundance of the BA-deconjugating enzyme bile salt hydrolase. In liver and small intestinal content of germ-free mice, there was a TCPOBOP-mediated increase in total, primary, and conjugated BAs corresponding with increased Cyp7a1 mRNA. Bifidobacterium, Dorea, Peptociccaceae, Anaeroplasma, and Ruminococcus positively correlated with T-UDCA in LIC, but negatively correlated with T-CDCA in serum. In conclusion, PXR and CAR activation downregulates BA-metabolizing bacteria in the intestine and modulates BA homeostasis in a gut microbiota-dependent manner.

Impaired Hepatic Phosphatidylcholine Synthesis Leads to Cholestasis in Mice Challenged With a High-Fat Diet

Phosphatidylethanolamine N-methyltransferase (PEMT) is a hepatic integral membrane protein localized to the endoplasmic reticulum (ER). PEMT catalyzes approximately 30% of hepatic phosphatidylcholine (PC) biosynthesis. Pemt^-/- mice fed a high-fat diet (HFD) develop steatohepatitis. Interestingly, portions of the ER located close to the canaliculus are enriched in PEMT. Phospholipid balance and asymmetrical distribution by adenosine triphosphatase phospholipid transporting 8B1 (ATP8B1) on the canalicular membrane is required for membrane integrity and biliary processes. We hypothesized that PEMT is an important supplier of PC to the canaliculus and that PEMT activity is critical for the maintenance of canalicular membrane integrity and bile formation following HFD feeding when there is an increase in overall hepatic PC demand. Pemt^+/+ and Pemt^-/- mice were fed a chow diet, an HFD, or a choline-supplemented HFD. Plasma and hepatic indices of liver function and parameters of bile formation were determined. Pemt^-/- mice developed cholestasis, i.e, elevated plasma bile acid (BA) concentrations and decreased biliary secretion rates of BAs and PC, during HFD feeding. The maximal BA secretory rate was reduced more than 70% in HFD-fed Pemt^-/- mice. Hepatic ABCB11/bile salt export protein, responsible for BA secretion, was decreased in Pemt^-/- mice and appeared to be retained intracellularly. Canalicular membranes of HFD-fed Pemt^-/- mice contained fewer invaginations and displayed a smaller surface area than Pemt^+/+ mice. Choline supplementation (CS) prevented and reversed the development of HFD-induced cholestasis. Conclusion: We propose that hepatic PC availability is critical for bile formation. Dietary CS might be a potential noninvasive therapy for a specific subset of patients with cholestasis.

Preclinical Evaluation of [18F]LCATD as a PET Tracer to Study Drug-Drug Interactions Caused by Inhibition of Hepatic Transporters

The bile acid analogue [¹⁸F]LCATD (LithoCholic Acid Triazole Derivative) is transported in vitro by hepatic uptake transporters such as OATP1B1 and NTCP and efflux transporter BSEP. In this in vivo “proof of principle” study, we tested if [¹⁸F]LCATD may be used to evaluate drug-drug interactions (DDIs) caused by inhibition of liver transporters. Hepatic clearance of [¹⁸F]LCATD in rats was significantly modified upon coadministration of rifamycin SV or sodium fusidate, which are known to inhibit clinically relevant uptake transporters (OATP1B1, NTCP) and canalicular hepatic transporters (BSEP) in humans. Treatment with rifamycin SV (total dose 62.5 mg·Kg⁻¹) reduced the maximum radioactivity of [¹⁸F]LCATD recorded in the liver from 14.2 ± 0.8% to 10.2 ± 0.9% and delayed t_{_max} by 90 seconds relative to control rats. AUC_{liver 0–5 min}, AUC_{bile 0–10 min} and hepatic uptake clearance CL_{uptake,in vivo} of rifamycin SV treated rats were significantly reduced, whereas AUC_{liver 0–30 min} was higher than in control rats. Administration of sodium fusidate (30 mg·Kg⁻¹) inhibited the liver uptake of [¹⁸F]LCATD, although to a lesser extent, reducing the maximum radioactivity in the liver to 11.5 ± 0.3%. These preliminary results indicate that [¹⁸F]LCATD may be a good candidate for future applications as an investigational tracer to evaluate altered hepatobiliary excretion as a result of drug-induced inhibition of hepatic transporters.

Investigation of Glycochenodeoxycholate Sulfate and Chenodeoxycholate Glucuronide as Surrogate Endogenous Probes for Drug Interaction Studies of OATP1B1 and OATP1B3 in Healthy Japanese Volunteers

PURPOSE

To assess the use of glycochenodeoxycholate-3-sulfate (GCDCA-S) and chenodeoxycholate 3- or 24-glucuronide (CDCA-3G or -24G) as surrogate endogenous substrates in the investigation of drug interactions involving OATP1B1 and OATP1B3.

METHODS

Uptake of GCDCA-S and CDCA-24G was examined in HEK293 cells transfected with cDNA for OATP1B1, OATP1B3, and NTCP and in cryopreserved human hepatocytes. Plasma concentrations of bile acids and their metabolites (GCDCA-S, CDCA-3G, and CDCA-24G) were determined by LC-MS/MS in eight healthy volunteers with or without administration of rifampicin (600 mg, po).

RESULTS

GCDCA-S and CDCA-24G were substrates for OATP1B1, OATP1B3, and NTCP. The uptake of [³H]atorvastatin, GCDCA-S, and CDCA-24G by human hepatocytes was significantly inhibited by both rifampicin and pioglitazone, whereas that of taurocholate was inhibited only by pioglitazone. Rifampicin elevated plasma concentrations of GCDCA-S more than those of other bile acids. The area under the plasma concentration-time curve for GCDCA-S was 20.3 times higher in rifampicin-treated samples. CDCA-24G could be detected only in plasma from the rifampicin-treatment phase, and CDCA-3G was undetectable in both phases.

CONCLUSIONS

We identified GCDCA-S and CDCA-24G as substrates of NTCP, OATP1B1, and OATP1B3. GCDCA-S is a surrogate endogenous probe for the assessment of drug interactions involving hepatic OATP1B1 and OATP1B3.

Activation of the Hypoxia Inducible Factor 1α Subunit Pathway in Steatotic Liver Contributes to Formation of Cholesterol Gallstones

BACKGROUND & AIMS

Hypoxia-inducible factor 1α subunit (HIF1A) is a transcription factor that controls the cellular response to hypoxia and is activated in hepatocytes of patients with nonalcoholic fatty liver disease (NAFLD). NAFLD increases the risk for cholesterol gallstone disease by unclear mechanisms. We studied the relationship between HIF1A and gallstone formation associated with liver steatosis.

METHODS

We performed studies with mice with inducible disruption of Hif1a in hepatocytes via a Cre adenoviral vector (inducible hepatocyte-selective HIF1A knockout [iH-HIFKO] mice), and mice without disruption of Hif1a (control mice). Mice were fed a diet rich in cholesterol and cholate for 1 or 2 weeks; gallbladders were collected and the number of gallstones was determined. Livers and biliary tissues were analyzed by histology, quantitative reverse-transcription polymerase chain reaction, immunohistochemistry, and immunoblots. We measured concentrations of bile acid, cholesterol, and phospholipid in bile and rates of bile flow. Primary hepatocytes and cholangiocytes were isolated and analyzed. HIF1A was knocked down in Hepa1-6 cells with small interfering RNAs. Liver biopsy samples from patients with NAFLD, with or without gallstones, were analyzed by quantitative reverse-transcription polymerase chain reaction.

RESULTS

Control mice fed a diet rich in cholesterol and cholate developed liver steatosis with hypoxia; levels of HIF1A protein were increased in hepatocytes around central veins and 90% of mice developed cholesterol gallstones. Only 20% of the iH-HIFKO mice developed cholesterol gallstones. In iH-HIFKO mice, the biliary lipid concentration was reduced by 36%, compared with control mice, and bile flow was increased by 35%. We observed increased water secretion from hepatocytes into bile canaliculi to mediate these effects, resulting in suppression of cholelithogenesis. Hepatic expression of aquaporin 8 (AQP8) protein was 1.5-fold higher in iH-HIFKO mice than in control mice. Under hypoxic conditions, cultured hepatocytes increased expression of Hif1a, Hmox1, and Vegfa messenger RNAs (mRNAs), and down-regulated expression of AQP8 mRNA and protein; AQP8 down-regulation was not observed in cells with knockdown of HIF1A. iH-HIFKO mice had reduced inflammation and mucin deposition in the gallbladder compared with control mice. Liver tissues from patients with NAFLD with gallstones had increased levels of HIF1A, HMOX1, and VEGFA mRNAs, compared with livers from patients with NAFLD without gallstones.

CONCLUSIONS

In steatotic livers of mice, hypoxia up-regulates expression of HIF1A, which reduces expression of AQP8 and concentrates biliary lipids via suppression of water secretion from hepatocytes. This promotes cholesterol gallstone formation. Livers from patients with NAFLD and gallstones express higher levels of HIF1A than livers from patients with NAFLD without gallstones.

A Predictive 3D Multi-Scale Model of Biliary Fluid Dynamics in the Liver Lobule

Bile, the central metabolic product of the liver, is transported by the bile canaliculi network. The impairment of bile flow in cholestatic liver diseases has urged a demand for insights into its regulation. Here, we developed a predictive 3D multi-scale model that simulates fluid dynamic properties successively from the subcellular to the tissue level. The model integrates the structure of the bile canalicular network in the mouse liver lobule, as determined by high-resolution confocal and serial block-face scanning electron microscopy, with measurements of bile transport by intravital microscopy. The combined experiment-theory approach revealed spatial heterogeneities of biliary geometry and hepatocyte transport activity. Based on this, our model predicts gradients of bile velocity and pressure in the liver lobule. Validation of the model predictions by pharmacological inhibition of Rho kinase demonstrated a requirement of canaliculi contractility for bile flow in vivo. Our model can be applied to functionally characterize liver diseases and quantitatively estimate biliary transport upon drug-induced liver injury.

Dioscin Protects ANIT-Induced Intrahepatic Cholestasis Through Regulating Transporters, Apoptosis and Oxidative Stress

Intrahepatic cholestasis, a clinical syndrome, is caused by excessive accumulation of bile acids in body and liver. Proper regulation of bile acids in liver cells is critical for liver injury. We previously reported the effects of dioscin against α-naphthylisothio- cyanate (ANIT)-induced cholestasis in rats. However, the pharmacological and mechanism data are limited. In our work, the animals of rats and mice, and Sandwich-cultured hepatocytes (SCHs) were caused by ANIT, and dioscin was used for the treatment. The results showed that dioscin markedly altered relative liver weights, restored ALT, AST, ALP, TBIL, GSH, GSH-Px, MDA, SOD levels, and rehabilitated ROS level and cell apoptosis. In mechanism study, dioscin not only significantly regulated the protein levels of Ntcp, OAT1, OCT1, Bsep and Mrp2 to accelerate bile acids excretion, but also regulated the expression levels of Bak, Bcl-xl, Bcl-2, Bax, Caspase 3 and Caspase 9 in vivo and in vitro to improve apoptosis. In addition, dioscin markedly inhibited PI3K/Akt pathway and up-regulated the levels of Nrf2, GCLc, GCLm, NQO1 and HO-1 against oxidative stress (OS) caused by bile acids. These results were further validated by inhibition of PI3K and Akt using the inhibitors of wortmannin and perifosine in SCHs. Our data showed that dioscin had good action against ANIT-caused intrahepatic cholestasis through regulating transporters, apoptosis and OS. This natural product can be considered as one active compound to treat intrahepatic cholestasis in the future.

Diverse relations between ABC transporters and lipids: An overview

It was first discovered in 1992 that P-glycoprotein (Pgp, ABCB1), an ATP binding cassette (ABC) transporter, can transport phospholipids such as phosphatidylcholine, -ethanolamine and -serine as well as glucosylceramide and glycosphingolipids. Subsequently, many other ABC transporters were identified to act as lipid transporters. For substrate transport by ABC transporters, typically a classic, alternating access model with an ATP-dependent conformational switch between a high and a low affinity substrate binding site is evoked. Transport of small hydrophilic substrates can easily be imagined this way, as the molecule can in principle enter and exit the transporter in the same orientation. Lipids on the other hand need to undergo a 180° degree turn as they translocate from one membrane leaflet to the other. Lipids and lipidated molecules are highly diverse, so there may be various ways how to achieve their flipping and flopping. Nonetheless, an increase in biophysical, biochemical and structural data is beginning to shed some light on specific aspects of lipid transport by ABC transporters. In addition, there is now abundant evidence that lipids affect ABC transporter conformation, dynamics as well as transport and ATPase activity in general. In this review, we will discuss different ways in which lipids and ABC transporters interact and how lipid translocation may be achieved with a focus on the techniques used to investigate these processes. This article is part of a Special Issue entitled: Lipid order/lipid defects and lipid-control of protein activity edited by Dirk Schneider.

Dynamics of hepatic and intestinal cholesterol and bile acid pathways: The impact of the animal model of estrogen deficiency and exercise training

Plasma cholesterol level is determined by a complex dynamics that involves transport lipoproteins which levels are tightly dependent on how the liver and the intestine regulate cholesterol and biliary acid metabolism. Regulation of cholesterol and biliary acids by the liver and the intestine is in turn coupled to a large array of enzymes and transporters that largely influence the inflow and the outflow of cholesterol and biliary acids through these organs. The activity of the key regulators of cholesterol and biliary acids may be influenced by several external factors such as pharmacological drugs and the nutritional status. In recent years, more information has been gathered about the impact of estrogens on regulation of cholesterol in the body. Exposure to high levels of estrogens has been reported to promote cholesterol gallstone formation and women are twice as likely as men to develop cholesterol gallstones. The impact of estrogen withdrawal, such as experienced by menopausal women, is therefore of importance and more information on how the absence of estrogens influence cholesterol regulation is started to come out, especially through the use of animal models. An interesting alternative to metabolic deterioration due to estrogen deficiency is exercise training. The present review is intended to summarize the present information that links key regulators of cholesterol and biliary acid pathways in liver and intestine to the absence of estrogens in an animal model and to discuss the potential role of exercise training as an alternative.

Role of enterohepatic recirculation in drug disposition: cooperation and complications

Enterohepatic recirculation (EHC) concerns many physiological processes and notably affects pharmacokinetic parameters such as plasma half-life and AUC as well as estimates of bioavailability of drugs. Also, EHC plays a detrimental role as the compounds/drugs are allowed to recycle. An in-depth comprehension of this phenomenon and its consequences on the pharmacological effects of affected drugs is important and decisive in the design and development of new candidate drugs. EHC of a compound/drug occurs by biliary excretion and intestinal reabsorption, sometimes with hepatic conjugation and intestinal deconjugation. EHC leads to prolonged elimination half-life of the drugs, altered pharmacokinetics and pharmacodynamics. Study of the EHC of any drug is complicated due to unavailability of the apposite model, sophisticated procedures and ethical concerns. Different in vitro and in vivo methods for studies in experimental animals and humans have been devised, each having its own merits and demerits. Involvement of the different transporters in biliary excretion, intra- and inter-species, pathological and biochemical variabilities obscure the study of the phenomenon. Modeling of drugs undergoing EHC has always been intricate and exigent models have been exploited to interpret the pharmacokinetic profiles of drugs witnessing multiple peaks due to EHC. Here, we critically appraise the mechanisms of bile formation, factors affecting biliary drug elimination, methods to estimate biliary excretion of drugs, EHC, multiple peak phenomenon and its modeling.

Disruption of BSEP Function in HepaRG Cells Alters Bile Acid Disposition and Is a Susceptive Factor to Drug-Induced Cholestatic Injury

In the present study, we characterized in vitro biosynthesis and disposition of bile acids (BAs) as well as hepatic transporter expression followed by ABCB11 (BSEP) gene knockout in HepaRG cells (HepaRG-KO cells). BSEP KO in HepaRG cells led to time-dependent BA accumulation, resulting in reduced biosynthesis of BAs and altered BA disposition. In HepaRG-KO cells, the expression of NTCP, OATP1B1, OATP2B1, BCRP, P-gp, and MRP2 were reduced, whereas MRP3 and OCT1 were up-regulated. As a result, BSEP KO altered the disposition of BAs and subsequently underwent adaptive regulations of BA synthesis and homeostasis to enable healthy growth of the cells. Although BSEP inhibitors caused no or slight increase of BAs in HepaRG wild type cells (HepaRG-WT cells), excessive intracellular accumulation of BAs was observed in HepaRG-KO cells exposed to bosentan and troglitazone, but not dipyridamole. LDH release in the medium was remarkably increased in HepaRG-KO cultures exposed to troglitazone (50 μM), suggesting drug-induced cellular injury. The results revealed that functional impairment of BSEP predisposes the cells to altered BA disposition and is a susceptive factor to drug-induced cholestatic injury. In total, BSEP inhibition might trigger the processes but is not a sole determinant of cholestatic cellular injury. As intracellular BA accumulation is determined by BSEP function and the subsequent adaptive gene regulation, assessment of intracellular BA accumulation in HepaRG-KO cells could be a useful approach to evaluate drug-induced liver injury (DILI) potentials of drugs that could disrupt other BA homeostasis pathways beyond BSEP inhibition.

Impaired Hepatic Uptake by Organic Anion-Transporting Polypeptides Is Associated with Hyperbilirubinemia and Hypercholanemia in Atp11c Mutant Mice

Biliary excretion of organic anions, such as bile acids (BAs), is the main osmotic driving force for bile formation, and its impairment induces intrahepatic cholestasis. We investigated the involvement of Atp11c in the hepatic transport of organic anions using Atp11c mutant mice, which exhibit hypercholanemia and hyperbilirubinemia. Pharmacokinetic analysis following a constant intravenous infusion in Atp11c mutant mice showed decreased hepatic sinusoidal uptake and intact biliary secretion of [(3)H]17β estradiol 17β-d-glucuronide. Consistent with this result, compared with cells and membranes from control mice, isolated hepatocytes, and liver plasma membranes from Atp11c mutant mice had a much lower uptake of [(3)H]17β estradiol 17β-d-glucuronide and expression of organic anion-transporting polypeptides, which are transporters responsible for hepatic uptake of unconjugated BAs and organic anions, including bilirubin glucuronides. Uptake of [(3)H]TC into hepatocytes and expression of Na(+)-taurocholate cotransporting polypeptide in liver plasma membranes, which mediates hepatic uptake of conjugated BAs, was also lower in the Atp11c mutant mice. Bile flow rate, biliary BA concentration, and expression of hepatobiliary transporters did not differ between Atp11c mutant mice and control mice. These results suggest that Atp11c mediates the transport of BAs and organic anions across the sinusoidal membrane, but not the canalicular membrane, by regulating the abundance of transporters. Atp11c is a candidate gene for genetically undiagnosed cases of hypercholanemia and hyperbilirubinemia, but not of intrahepatic cholestasis. This gene may influence the pharmacological and adverse effect of drugs because organic anion-transporting polypeptides regulate their systemic exposure.

Regulation of plasma membrane localization of the Na+-taurocholate cotransporting polypeptide (Ntcp) by hyperosmolarity and tauroursodeoxycholate

In perfused rat liver, hepatocyte shrinkage induces a Fyn-dependent retrieval of the bile salt export pump (Bsep) and multidrug resistance-associated protein 2 (Mrp2) from the canalicular membrane (Cantore, M., Reinehr, R., Sommerfeld, A., Becker, M., and Häussinger, D. (2011) J. Biol. Chem. 286, 45014-45029) leading to cholestasis. However little is known about the effects of hyperosmolarity on short term regulation of the Na(+)-taurocholate cotransporting polypeptide (Ntcp), the major bile salt uptake system at the sinusoidal membrane of hepatocytes. The aim of this study was to analyze hyperosmotic Ntcp regulation and the underlying signaling events. Hyperosmolarity induced a significant retrieval of Ntcp from the basolateral membrane, which was accompanied by an activating phosphorylation of the Src kinases Fyn and Yes but not of c-Src. Hyperosmotic internalization of Ntcp was sensitive to SU6656 and PP-2, suggesting that Fyn mediates Ntcp retrieval from the basolateral membrane. Hyperosmotic internalization of Ntcp was also found in livers from wild-type mice but not in p47(phox) knock-out mice. Tauroursodeoxycholate (TUDC) and cAMP reversed hyperosmolarity-induced Fyn activation and triggered re-insertion of the hyperosmotically retrieved Ntcp into the membrane. This was associated with dephosphorylation of the Ntcp on serine residues. Insertion of Ntcp by TUDC was sensitive to the integrin inhibitory hexapeptide GRGDSP and inhibition of protein kinase A. TUDC also reversed the hyperosmolarity-induced retrieval of bile salt export pump from the canalicular membrane. These findings suggest a coordinated and oxidative stress- and Fyn-dependent retrieval of sinusoidal and canalicular bile salt transport systems from the corresponding membranes. Ntcp insertion was also identified as a novel target of β1-integrin-dependent TUDC action, which is frequently used in the treatment of cholestatic liver disease.

Effect of Yin-Zhi-Huang on up-regulation of Oatp2, Ntcp, and Mrp2 proteins in estrogen-induced rat cholestasis

CONTEXT

Yin-Zhi-Huang (YZH), a prescription of traditional Chinese medicine, is widely used to treat neonatal jaundice or cholestasis.

OBJECTIVE

This study investigates the regulatory effect of YZH on the localization and expression of organic anion transporting polypeptides 2 (Oatp2), Na(+)-taurocholate co-transporting polypeptide (Ntcp), multidrug-resistance-associated protein 2 (Mrp2), and bile salt export pump (Bsep) in estrogen-induced cholestasis rats.

MATERIAL AND METHODS

Cholestasis model rats were induced via subcutaneous injection of estradiol benzoate (EB, 5 mg/kg/d) for 5 d. Other EB-induced rats were treated with saline (2 ml) or YZH (1.5 g/kg, two times a day) for 7, 14, and 21 d. The biochemical and pathologic examinations were performed. Moreover, the localization and expression of Oatp2, Ntcp, Mrp2, and Bsep were determined by immunohistochemisty and Western blotting, respectively.

RESULTS

YZH treatment could significantly decrease the serum total bile acids (TBA) (4.9 ± 0.6-2.8 ± 0.8) and direct bilirubin (DBIL) (2.6 ± 0.7-1.0 ± 0.1) levels, improve the histological disorganization, and, respectively, increase the expression of Oatp2 and Ntcp by 46% and 28% compared with saline-treated (p < 0.05) rats at 14 d. The expression of Mrp2 increased by 45% was observed in YZH treated compared with saline-treated (p < 0.05) rats at 7 d. However, there was a little change in the expression of Bsep (p > 0.05) after YZH treatment for 7, 14, and 21 d.

DISCUSSION AND CONCLUSION

In conclusion, the therapeutic effect of YZH to cholestasis could be attributed to the regulation of Oatp2, Ntcp, Mrp2, and Bsep.

New insights in the biology of ABC transporters ABCC2 and ABCC3: impact on drug disposition

INTRODUCTION

For the elimination of environmental chemicals and metabolic waste products, the body is equipped with a range of broad specificity transporters that are present in excretory organs as well as in several epithelial blood-tissue barriers.

AREAS COVERED

ABCC2 and ABCC3 (also known as MRP2 and MRP3) mediate the transport of various conjugated organic anions, including many drugs, toxicants and endogenous compounds. This review focuses on the physiology of these transporters, their roles in drug disposition and how they affect drug sensitivity and toxicity. It also examines how ABCC2 and ABCC3 are coordinately regulated at the transcriptional level by members of the nuclear receptor (NR) family of ligand-modulated transcription factors and how this can be therapeutically exploited.

EXPERT OPINION

Mutations in both ABCC2 and ABCC3 have been associated with changes in drug disposition, sensitivity and toxicity. A defect in ABCC2 is associated with Dubin-Johnson syndrome, a recessively inherited disorder characterized by conjugated hyperbilirubinemia. Pharmacological manipulation of the activity of these transporters can potentially improve the pharmacokinetics and thus therapeutic activity of substrate drugs but also affect the physiological function of these transporters and consequently ameliorate associated disease states.

Atorvastatin induces bile acid-synthetic enzyme Cyp7a1 by suppressing FXR signaling in both liver and intestine in mice

Statins are effective cholesterol-lowering drugs to treat CVDs. Bile acids (BAs), the end products of cholesterol metabolism in the liver, are important nutrient and energy regulators. The present study aims to investigate how statins affect BA homeostasis in the enterohepatic circulation. Male C57BL/6 mice were treated with atorvastatin (100 mg/kg/day po) for 1 week, followed by BA profiling by ultra-performance LC-MS/MS. Atorvastatin decreased BA pool size, mainly due to less BA in the intestine. Surprisingly, atorvastatin did not alter total BAs in the serum or liver. Atorvastatin increased the ratio of 12α-OH/non12α-OH BAs. Atorvastatin increased the mRNAs of the BA-synthetic enzymes cholesterol 7α-hydroxylase (Cyp7a1) (over 10-fold) and cytochrome P450 27a1, the BA uptake transporters Na⁺/taurocholate cotransporting polypeptide and organic anion transporting polypeptide 1b2, and the efflux transporter multidrug resistance-associated protein 2 in the liver. Noticeably, atorvastatin suppressed the expression of BA nuclear receptor farnesoid X receptor (FXR) target genes, namely small heterodimer partner (liver) and fibroblast growth factor 15 (ileum). Furthermore, atorvastatin increased the mRNAs of the organic cation uptake transporter 1 and cholesterol efflux transporters Abcg5 and Abcg8 in the liver. The increased expression of BA-synthetic enzymes and BA transporters appear to be a compensatory response to maintain BA homeostasis after atorvastatin treatment. The Cyp7a1 induction by atorvastatin appears to be due to suppressed FXR signaling in both the liver and intestine.

Intestinal transport and metabolism of bile acids

In addition to their classical roles as detergents to aid in the process of digestion, bile acids have been identified as important signaling molecules that function through various nuclear and G protein-coupled receptors to regulate a myriad of cellular and molecular functions across both metabolic and nonmetabolic pathways. Signaling via these pathways will vary depending on the tissue and the concentration and chemical structure of the bile acid species. Important determinants of the size and composition of the bile acid pool are their efficient enterohepatic recirculation, their host and microbial metabolism, and the homeostatic feedback mechanisms connecting hepatocytes, enterocytes, and the luminal microbiota. This review focuses on the mammalian intestine, discussing the physiology of bile acid transport, the metabolism of bile acids in the gut, and new developments in our understanding of how intestinal metabolism, particularly by the gut microbiota, affects bile acid signaling.

Clinical application of transcriptional activators of bile salt transporters

Hepatobiliary bile salt (BS) transporters are critical determinants of BS homeostasis controlling intracellular concentrations of BSs and their enterohepatic circulation. Genetic or acquired dysfunction of specific transport systems causes intrahepatic and systemic retention of potentially cytotoxic BSs, which, in high concentrations, may disturb integrity of cell membranes and subcellular organelles resulting in cell death, inflammation and fibrosis. Transcriptional regulation of canalicular BS efflux through bile salt export pump (BSEP), basolateral elimination through organic solute transporters alpha and beta (OSTα/OSTβ) as well as inhibition of hepatocellular BS uptake through basolateral Na(+)-taurocholate cotransporting polypeptide (NTCP) represent critical steps in protection from hepatocellular BS overload and can be targeted therapeutically. In this article, we review the potential clinical implications of the major BS transporters BSEP, OSTα/OSTβ and NTCP in the pathogenesis of hereditary and acquired cholestatic syndromes, provide an overview on transcriptional control of these transporters by the key regulatory nuclear receptors and discuss the potential therapeutic role of novel transcriptional activators of BS transporters in cholestasis.

Key discoveries in bile acid chemistry and biology and their clinical applications: history of the last eight decades

During the last 80 years there have been extraordinary advances in our knowledge of the chemistry and biology of bile acids. We present here a brief history of the major achievements as we perceive them. Bernal, a physicist, determined the X-ray structure of cholesterol crystals, and his data together with the vast chemical studies of Wieland and Windaus enabled the correct structure of the steroid nucleus to be deduced. Today, C24 and C27 bile acids together with C27 bile alcohols constitute most of the bile acid "family". Patterns of bile acid hydroxylation and conjugation are summarized. Bile acid measurement encompasses the techniques of GC, HPLC, and MS, as well as enzymatic, bioluminescent, and competitive binding methods. The enterohepatic circulation of bile acids results from vectorial transport of bile acids by the ileal enterocyte and hepatocyte; the key transporters have been cloned. Bile acids are amphipathic, self-associate in solution, and form mixed micelles with polar lipids, phosphatidylcholine in bile, and fatty acids in intestinal content during triglyceride digestion. The rise and decline of dissolution of cholesterol gallstones by the ingestion of 3,7-dihydroxy bile acids is chronicled. Scientists from throughout the world have contributed to these achievements.

Physiological and molecular biochemical mechanisms of bile formation

This review considers the physiological and molecular biochemical mechanisms of bile formation. The composition of bile and structure of a bile canaliculus, biosynthesis and conjugation of bile acids, bile phospholipids, formation of bile micellar structures, and enterohepatic circulation of bile acids are described. In general, the review focuses on the molecular physiology of the transporting systems of the hepatocyte sinusoidal and apical membranes. Knowledge of physiological and biochemical basis of bile formation has implications for understanding the mechanisms of development of pathological processes, associated with diseases of the liver and biliary tract.

Thermodynamic and solution state NMR characterization of the binding of secondary and conjugated bile acids to STARD5

STARD5 is a member of the STARD4 sub-family of START domain containing proteins specialized in the non-vesicular transport of lipids and sterols. We recently reported that STARD5 binds primary bile acids. Herein, we report on the biophysical and structural characterization of the binding of secondary and conjugated bile acids by STARD5 at physiological concentrations. We found that the absence of the 7α-OH group and its epimerization increase the affinity of secondary bile acids for STARD5. According to NMR titration and molecular modeling, the affinity depends mainly on the number and positions of the steroid ring hydroxyl groups and to a lesser extent on the presence or type of bile acid side-chain conjugation. Primary and secondary bile acids have different binding modes and display different positioning within the STARD5 binding pocket. The relative STARD5 affinity for the different bile acids studied is: DCA>LCA>CDCA>GDCA>TDCA>CA>UDCA. TCA and GCA do not bind significantly to STARD5. The impact of the ligand chemical structure on the thermodynamics of binding is discussed. The discovery of these new ligands suggests that STARD5 is involved in the cellular response elicited by bile acids and offers many entry points to decipher its physiological role.

High doses of ursodeoxycholic acid up-regulate the expression of placental breast cancer resistance protein in patients affected by intrahepatic cholestasis of pregnancy

Background

Ursodeoxycholic acid (UDCA) administration in intrahepatic cholestasis of pregnancy (ICP) induces bile acids (BA) efflux from the foetal compartment, but the molecular basis of this transplacental transport is only partially defined.

Aim

To determine if placental breast cancer resistance protein (BCRP), able to transport BA, is regulated by UDCA in ICP.

Methods

32 pregnant women with ICP (14 untreated, 34.9±5.17 years; 18 treated with UDCA - 25 mg/Kg/day, 32.7±4.62 years,) and 12 healthy controls (33.4±3.32 years) agreed to participate in the study. Placentas were obtained at delivery and processed for membrane extraction. BCRP protein expression was evaluated by immunoblotting techniques and chemiluminescence quantified with a luminograph measuring emitted photons; mRNA expression with real time PCR. Statistical differences between groups were evaluated by ANOVA with Dunn’s Multiple Comparison test.

Results

BCRP was expressed only on the apical membrane of the syncytiotrophoblast. A significant difference was observed among the three groups both for mRNA (ANOVA, p = 0.0074) and protein (ANOVA, p<0.0001) expression. BCRP expression was similar in controls and in the untreated ICP group. UDCA induced a significant increase in placental BCRP mRNA and protein expression compared to controls (350.7±106.3 vs 100±18.68% of controls, p<0.05 and 397.8±56.02 vs 100±11.44% of controls, p<0.001, respectively) and untreated ICP (90.29±17.59% of controls, p<0.05 and 155.0±13.87%, p<0.01).

Conclusion

Our results confirm that BCRP is expressed only on the apical membrane of the syncytiotrophoblast and show that ICP treatment with high dose UDCA significantly upregulates placental BCRP expression favouring BA efflux from the foetal compartment.

The vesicular transport assay: validated in vitro methods to study drug-mediated inhibition of canalicular efflux transporters ABCB11/BSEP and ABCC2/MRP2

The canalicular membrane of hepatocytes contains several transport proteins that use the energy of ATP to efflux potentially toxic molecules to the bile. Probably the two most important proteins at this location are MRP2 and BSEP, which transport phase II conjugates of xenobiotics and endobiotics and conjugated bile salts, respectively. The impaired function of either of these transporter proteins reduces the clearance of the toxic conjugates, resulting in their accumulation in the hepatocytes and eventually the plasma. Conjugated bile salts and phase II metabolites are compounds with low passive permeability; therefore, the most commonly used test system to investigate MRP2- and BSEP-mediated transport processes is the vesicular transport assay. The concentration of probe substrates and inhibitors used in the experiment is close to their free concentration in the hepatocytes, providing an advantage when calculating kinetic parameters (K(m), K(i), V(max)). The protocols aim to assist scientists to set up a transport assay for a known or potential substrate and test small molecule inhibition of the transporters.

Short-term feedback regulation of bile salt uptake by bile salts in rodent liver

The sodium taurocholate cotransporting polypeptide (Ntcp) is the major bile salt uptake transporter at the sinusoidal membrane of hepatocytes. Short-term feedback regulation of Ntcp by primary bile salts has not yet been investigated in vivo. Subcellular localization of Ntcp was analyzed in Ntcp-transfected HepG2-cells by flow cytometry and in immunofluorescence images from tissue sections by a new automated image analysis method. Net bile salt uptake was investigated in perfused rat liver by a pulse chase technique. In Flag-Ntcp-EGFP (enhanced green fluorescent protein) expressing HepG2-cells, taurochenodeoxycholate (TCDC), but not taurocholate (TC), induced endocytosis of Ntcp. TCDC, but not TC, caused significant internalization of Ntcp in perfused rat livers, as shown by an increase in intracellular Ntcp immunoreactivity, whereas Bsep distribution remained unchanged. These results correlate with functional studies. Rat livers were continuously perfused with 100 μmol/L of TC. 25 μmol/L of TCDC, taurodeoxycholate (TDC), tauroursodeoxycholate (TUDC), or TC were added for 30 minutes, washed out, followed by a pulse of (3) [H]-TC. TCDC, but not TDC, TUDC, or TC significantly increased the amount of (3) [H]-TC in the effluent, indicating a reduced sinusoidal net TC uptake. This effect was sensitive to chelerythrine (protein kinase C inhibitor) and cypermethrin (protein phosphatase 2B inhibitor). Phosphoinositide 3-kinase (PI3K) inhibitors had an additive effect, whereas Erk1/2 (extracellular signal activated kinase 1/2), p38MAPK, protein phosphatase 1/2A (PP1/2A), and reactive oxygen species (ROS) were not involved.

CONCLUSION

TCDC regulates bile salt transport at the sinusoidal membrane by protein kinase C- and protein phosphatase 2B-mediated retrieval of Ntcp from the plasma membrane. During increased portal bile salt load this mechanism may adjust bile salt uptake along the acinus and protect periportal hepatocytes from harmful bile salt concentrations.

Bilirubin and beyond: a review of lipid status in Gilbert's syndrome and its relevance to cardiovascular disease protection

Gilbert's syndrome (GS) is characterized by a benign, mildly elevated bilirubin concentration in the blood. Recent reports show clear protection from cardiovascular disease in this population. Protection of lipids, proteins and other macromolecules from oxidation by bilirubin represents the most commonly accepted mechanism contributing to protection in this group. However, a recent meta-analysis estimated that bilirubin only accounts for ~34% of the cardioprotective effects within analysed studies. To reveal the additional contributing variables we have explored circulating cholesterol and triacylglycerol concentrations, which appear to be decreased in hyperbilirubinemic individuals/animals, and are accompanied by lower body mass index in highly powered studies. These results suggest that bilirubin could be responsible for the development of a lean and hypolipidemic state in GS. Here we also discuss the possible contributing mechanisms that might reduce circulating cholesterol and triacylglycerol concentrations in individuals with syndromes affecting bilirubin metabolism/excretion, which we hope will stimulate future research in the area. In summary, this article is the first review of lipid status in animal and human studies of hyperbilirubinemia and explores possible mechanisms that could contribute to lowering circulating lipid parameters and further explain cardiovascular protection in Gilbert's syndrome.